In known fuel injection systems, a high pressure fuel injection pump is arranged to supply fuel from a pumping chamber to an associated injector arranged downstream of the pumping chamber. The injector may be arranged in a common housing with the pump or may be separated from the pump by a dedicated injector supply line. The pump includes a pumping plunger which is reciprocal within a plunger bore to perform a pumping cycle including a pumping stroke and a return stroke. During the pumping stroke, the pumping plunger is driven by means of a cam drive arrangement to reduce the volume of the pumping chamber so that fuel within the pumping chamber is pressurised. Pressurised fuel is delivered from the pumping chamber to the injector through a pump outlet via an outlet valve. During the return stroke (or filling stroke), the pumping plunger is withdrawn from the plunger bore under a return spring force so as to increase the volume of the pumping chamber. Fuel fills the pumping chamber through a fill/spill port in communication with a low pressure reservoir during that part of the return stroke for which the fill/spill port is open.
It is particularly important to be able to control accurately the timing and quantity of fuel delivery to the engine cylinder so as to improve fuel economy and engine emissions. For this purpose it is known to provide the pumping plunger with control features so as to provide control of the quantity and timing of fuel that is delivered during the pumping cycle. By way of example, the plunger defines an upper control edge on its end face in the pumping chamber and is provided with a helical groove on its side face to define a lower control edge. During the pumping stroke, pressurisation of fuel in the pumping chamber is commenced when the upper control edge closes the fill/spill port into the pumping chamber. Pressurisation is terminated when the pumping plunger has moved sufficiently far through the pumping stroke for the lower control edge defined by the helical groove to open communication between the pumping chamber and the fill/spill port and, hence, the low pressure drain.
The angular position of the pumping plunger determines the point in the pumping stroke at which the upper control edge of the pumping plunger closes the fill/spill port, thus starting fuel pressurisation earlier, or later, in the pumping stroke. Consequently, this varies the point in the injection cycle at which injection is initiated. The angular position of the pumping plunger also determines the point in the pumping cycle at which the helical groove registers with the fill/spill port, thus terminating pressurisation (and hence injection) earlier, or later, in the pumping stroke. The variation of the effective stroke between the upper control edge of the pumping plunger and the lower control edge of the helical groove varies the delivered fuel quantity. During the effective stroke, the registration of the outer surface of the pumping plunger with the fill/spill port closes communication between the fill/spill port and the low pressure drain.
To provide further adjustment of the timing of initiation of fuel delivery, it is known to provide the pump with a timing advance arrangement. A cam follower arrangement is typically disposed between the pumping plunger and the cam drive arrangement, the cam follower arrangement including a timing advance piston which is movable in response to fluid pressure controlled by an advance control. The advance piston is mounted within a bore provided in a cam follower component, such as a tappet. By pressurising the advance piston, it is displaced outwardly from the rotational axis of the cam which, in turn, displaces the pumping plunger further away from the rotational axis of the cam. The position of the pumping plunger within the plunger bore determines fuel injection timing, as described above, and so the advance piston provides a means for adjusting the timing, depending on whether the advance piston is advanced or retracted under the advance control. The use of a timing advance device of the aforementioned type is known to have particular benefits when running under cold conditions as it allows white smoke emissions to be decreased.
It has been observed that in fuel pumps provided with a timing advance device as described above, there is a tendency for the advance piston and the tappet bore to become misaligned during running due to the poor length to diameter ratio of the piston. Also, any concentricity misalignment of the advance piston relative to the tappet bore will affect the leakage rate through the clearance between the components, giving an undesirable performance variability between different units.
It is an object of the present invention to provide a timing advance device for use in a fuel pump which overcomes or alleviates the aforementioned problems.